4,326 research outputs found
Properties of Long Gamma-Ray Burst Host Galaxies in Cosmological Simulation
We use galaxy catalogues constructed by combining high-resolution N-body
simulations with semi-analytic models of galaxy formation to study the
properties of Long Gamma-Ray Burst (LGRB) host galaxies. We assume that LGRBs
originate from the death of massive young stars and analyse how results are
affected by different metallicity constraints on the progenitor stars. As
expected, the host sample with no metallicity restriction on the progenitor
stars provides a perfect tracer of the cosmic star formation history. When
LGRBs are required to be generated by low-metallicity stars, they trace a
decreasing fraction of the cosmic star formation rate at lower redshift, as a
consequence of the global increase in metallicity. We study the properties of
host galaxies up to high redshift (~9), finding that they typically have
low-metallicity (Z<0.5 Z_sun) and that they are small (M<10^9 M_sun), bluer and
younger than the average galaxy population, in agreement with observational
data. They are also less clustered than typical L_* galaxies in the Universe,
and their descendents are massive, red and reside in groups of galaxies with
halo mass between 10^{13} M_sun to 10^{14} M_sun.Comment: The paper contains 14 figures and 2 tables. Accepted MNRA
Exploring Galaxy Formation Models and Cosmologies with Galaxy Clustering
Using N-body simulations and galaxy formation models, we study the galaxy
stellar mass correlation and the two-point auto-correlation. The simulations
are run with cosmological parameters from the WMAP first, third and seven year
results, which mainly differ in the perturbation amplitude of \sigma_{8}. The
stellar mass of galaxies are determined using either a semi-analytical galaxy
formation model or a simple empirical abundance matching method. Compared to
the SDSS DR7 data at z=0 and the DEEP2 results at z=1, we find that the
predicted galaxy clusterings from the semi-analytical model are higher than the
data at small scales, regardless of the adopted cosmology. Conversely, the
abundance matching method predicts good agreement with the data at both z=0 and
z=1 for high \sigma_8 cosmologies (WMAP1 & WMAP7), but the predictions from a
low \sigma_8 cosmology (WMAP3) are significantly lower than the data at z=0. We
find that the excess clustering at small-scales in the semi-analytical model
mainly arises from satellites in massive haloes, indicating that either the
star formation is too efficient in low-mass haloes or tidal stripping is too
inefficient at high redshift. Our results show that galaxy clustering is
strongly affected by the models for galaxy formation, thus can be used to
constrain the baryonic physics. The weak dependence of galaxy clustering on
cosmological parameters makes it difficult to constrain the WMAP1 and WMAP7
cosmologies.Comment: 8 pages, 7 figures. Accepted to MNRA
Autocorrelations of stellar light and mass at z~0 and ~1: From SDSS to DEEP2
We present measurements of projected autocorrelation functions w_p(r_p) for
the stellar mass of galaxies and for their light in the U, B and V bands, using
data from the third data release of the DEEP2 Galaxy Redshift Survey and the
final data release of the Sloan Digital Sky Survey (SDSS). We investigate the
clustering bias of stellar mass and light by comparing these to projected
autocorrelations of dark matter estimated from the Millennium Simulations (MS)
at z=1 and 0.07, the median redshifts of our galaxy samples. All of the
autocorrelation and bias functions show systematic trends with spatial scale
and waveband which are impressively similar at the two redshifts. This shows
that the well-established environmental dependence of stellar populations in
the local Universe is already in place at z=1. The recent MS-based galaxy
formation simulation of Guo et al. (2011) reproduces the scale-dependent
clustering of luminosity to an accuracy better than 30% in all bands and at
both redshifts, but substantially overpredicts mass autocorrelations at
separations below about 2 Mpc. Further comparison of the shapes of our stellar
mass bias functions with those predicted by the model suggests that both the
SDSS and DEEP2 data prefer a fluctuation amplitude of sigma_8 0.8 rather than
the sigma_8=0.9 assumed by the MS.Comment: 10 pages, 4 figures, accepted for publication in Monthly Notices,
minor revisions in tex
The distribution of stellar mass in the low-redshift Universe
We use a complete and uniform sample of almost half a million galaxies from
the Sloan Digital Sky Survey to characterise the distribution of stellar mass
in the low-redshift Universe. Galaxy abundances are well determined over almost
four orders of magnitude in stellar mass, and are reasonably but not perfectly
fit by a Schechter function with characteristic stellar mass m* = 6.7 x 10^10
M_sun and with faint-end slope \alpha = -1.155. For a standard cosmology and a
standard stellar Initial Mass Function, only 3.5% of the baryons in the
low-redshift Universe are locked up in stars. The projected autocorrelation
function of stellar mass is robustly and precisely determined for r_p < 30
Mpc/h. Over the range 10 kpc/kpc < r_p < 10 Mpc/h it is extremely well
represented by a power law. The corresponding three-dimensional autocorrelation
function is \xi*(r) = (r/6.1 Mpc/h)^{-1.84}. Relative to the dark matter, the
bias of the stellar mass distribution is approximately constant on large
scales, but varies by a factor of five for r_p < 1 Mpc/h. This behaviour is
approximately but not perfectly reproduced by current models for galaxy
formation in the concordance LCDM cosmology. Detailed comparison suggests that
a fluctuation amplitude \sigma_8 ~ 0.8 is preferred to the somewhat larger
value adopted in the Millennium Simulation models with which we compare our
data. This comparison also suggests that observations of stellar mass
autocorrelations as a function of redshift might provide a powerful test for
the nature of Dark Energy.Comment: 12 pages, 11 figures, accepted for publication in Monthly Notices,
two appendices added to explore possible systematic biases due to the stellar
mass definition and surface density limit
New improved Moser-Trudinger inequalities and singular Liouville equations on compact surfaces
We consider a singular Liouville equation on a compact surface, arising from
the study of Chern-Simons vortices in a self dual regime. Using new improved
versions of the Moser-Trudinger inequalities (whose main feature is to be
scaling invariant) and a variational scheme, we prove new existence results.Comment: to appear in GAF
The metallicity properties of simulated long-GRB galaxy hosts and the Fundamental Metallicity Relation
We study the implication of the collapsar model for Long Gamma-Ray Bursts
(LGRBs) on the metallicity properties of the host galaxies, by combining
high-resolution N-body simulations with semi-analytic models of galaxy
formation. The cosmological model that we use reproduces the Fundamental
Metallicity Relation recently discovered for the SDSS galaxies, whereby the
metallicity decreases with increasing Star Formation Rate for galaxies of a
given stellar mass. We select host galaxies housing pockets of gas-particles,
young and with different thresholds in metallicities, that can be sites of LRGB
events, according to the collapsar model. The simulated samples are compared
with 18 observed LGRB hosts in the aim at discriminating whether the
metallicity is a primary parameter. We find that a threshold in metallicity for
the LGRB progenitors, within the model galaxies, is not necessary in order to
reproduce the observed distribution of host metallicities. The low
metallicities of observed LGRB hosts is a consequence of the high star
formation environment. The star formation rate appears to be the primary
parameter to generate a burst event. Finally, we show that only a few LGRBs are
observed in massive, highly extincted galaxies, while these galaxies are
expected to produce many such events. We identify these missing events with the
fraction of dark LGRBs.Comment: 9 pages, 5 figures, submitted MNRA
Severity, timing, and structure of disability
Objectives: Severity and timing are key aspects of disability experience for individuals. They also generate a populationâs disability structure (prevalence, counts, patterns). We study links among severity, duration, and structure for community-dwelling adults in the US.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43488/1/038_2004_Article_3058.pd
Worry Is Good for Breast Cancer Screening: A Study of Female Relatives from the Ontario Site of the Breast Cancer Family Registry
Background. Few prospective studies have examined associations between breast cancer worry and screening behaviours in women with elevated breast cancer risks based on family history. Methods. This study included 901 high familial risk women, aged 23â71 years, from the Ontario site of the Breast Cancer Family Registry. Self-reported breast screening behaviours at year-one followup were compared between women at low (N=305), medium (N=433), and high (N=163) levels of baseline breast cancer worry using logistic regression. Nonlinear relationships were assessed using likelihood ratio tests. Results. A significant non-linear inverted âUâ relationship was observed between breast cancer worry and mammography screening (P=0.034) for all women, where women at either low or high worry levels were less likely than those at medium to have a screening mammogram. A similar significant non-linear inverted âUâ relationship was also found among all women and women at low familial risk for worry and screening clinical breast examinations (CBEs). Conclusions. Medium levels of cancer worries predicted higher rates of screening mammography and CBE among high-risk women
Linking haloes to galaxies: how many halo properties are needed?
Recent studies emphasize that an empirical relation between the stellar mass
of galaxies and the mass of their host dark matter subhaloes can predict the
clustering of galaxies and its evolution with cosmic time. In this paper we
study the assumptions made by this methodology using a semi-analytical model
(SAM). To this end, we randomly swap between the locations of model galaxies
within a narrow range of subhalo mass (M_infall). We find that shuffled samples
of galaxies have different auto-correlation functions in comparison with the
original model galaxies. This difference is significant even if central and
satellite galaxies are allowed to follow a different relation between M_infall
and stellar mass, and can reach a factor of 2 for massive galaxies at redshift
zero. We analyze three features within SAMs that contribute to this effect: a)
The relation between stellar mass and subhalo mass evolves with redshift for
central galaxies, affecting satellite galaxies at the time of infall. b) The
stellar mass of galaxies falling into groups and clusters at high redshift is
different from the mass of central galaxies at the same time. c) The stellar
mass growth for satellite galaxies after infall can be significant and depends
on the infall redshift and the group mass. We show that the above is true for
differing SAMs, and that the effect is sensitive to the treatment of dynamical
friction and stripping of gas in satellite galaxies. We find that by using the
FoF group mass at redshift zero in addition to M_infall, an empirical model is
able to accurately reproduce the clustering properties of galaxies. On the
other hand, using the infall redshift as a second parameter does not yield as
good results because it is less correlated with stellar mass. Our analysis
indicates that environmental processes are important for modeling the
clustering and abundance of galaxies. (Abridged)Comment: Accepted for publication in MNRAS, minor changes from version
Criteria for arrhythmogenicity in genetically-modified Langendorff-perfused murine hearts modelling the congenital long QT syndrome type 3 and the Brugada syndrome
The experiments investigated the applicability of two established criteria for arrhythmogenicity in Scn5a+/Î and Scn5a+/â murine hearts modelling the congenital long QT syndrome type 3 (LQT3) and the Brugada syndrome (BrS). Monophasic action potentials (APs) recorded during extrasystolic stimulation procedures from Langendorff-perfused control hearts and hearts treated with flecainide (1Â ÎŒM) or quinidine (1 or 10Â ÎŒM) demonstrated that both agents were pro-arrhythmic in wild-type (WT) hearts, quinidine was pro-arrhythmic in Scn5a+/Î hearts, and that flecainide was pro-arrhythmic whereas quinidine was anti-arrhythmic in Scn5a+/â hearts, confirming clinical findings. Statistical analysis confirmed a quadratic relationship between epicardial and endocardial AP durations (APDs) in WT control hearts. However, comparisons between plots of epicardial against endocardial APDs and this reference curve failed to correlate with arrhythmogenicity. Restitution curves, relating APD to diastolic interval (DI), were then constructed for the first time in a murine system and mono-exponential growth functions fitted to these curves. Significant (Pâ<â0.05) alterations in the DI at which slopes equalled unity, an established indicator of arrhythmogenicity, now successfully predicted the presence or absence of arrhythmogenicity in all cases. We thus associate changes in the slopes of restitution curves with arrhythmogenicity in models of LQT3 and BrS
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